A: Vahid Ebadat, Stonehouse Process Safety, says:
We’re surrounded by electrostatic effects every day. If you’ve ever had a shock when you touched an elevator button or heard crackling sounds when you brush your hair, then you’ve experienced static electricity. With static so prevalent at home, we can’t really expect it to be absent from the workplace. The main difference, of course, is that in the workplace, static happens at a larger scale and with increased energy, and in some manufacturing environments, we also find flammable atmospheres created by combustible powders, explosible dust clouds, or low flash-point liquids. Static electricity and a flammable atmosphere can be an ominous combination.
As an electrostatic hazard consultant, I spend many hours advising clients on precautions against static electricity and conduct focused electrostatic hazard assessments in manufacturing plants. An important observation from my experience is that it’s quite usual for electrostatic phenomena to be present in a facility. Sometimes there are warning signs before an event. Such electrostatic phenomena must be reported and investigated with urgency, as they can be precursors to a fire or explosion. Here are some tips on identifying the sights, sounds, and experiences of static electricity in a plant and the early warning signs that must be investigated.
From pharmaceuticals to food processing to plastics manufacturing, you don’t need to look far to find electrostatic phenomena. These phenomena are listed in Table I and can be grouped by class as visible, audible, or experiential. Within these classifications, we can observe subclasses as flashes or glows; hear sounds as clicks, hisses, or cracks; and we can experience phenomena such as electric shock, tingling, strange powder flow or adhesion, or even plant damage. Let’s take a closer look.
Generation. In a plant, powder movement can generate huge quantities of static electricity. Plant personnel can also generate static as they go about their work by moving equipment, materials, or simply walking about. Operating equipment can also generate charge on the equipment or processed materials.
In most cases of an electrostatic charge, there’s movement between two surfaces that come into contact with each other and then separate. Most, but not all, static electricity in a plant is generated by so-called triboelectrification or flow electrification processes, which is the constant making and breaking of surface contact between materials. Powder movement, equipment with moving parts, and plant personnel are all subjected to surface contact and separation in some form. (Contact + Separation = Static charge generation)
Accumulation. However, static charge generation isn’t enough to create the effects described in the table. The generated charge has to build up or accumulate on something before we see the effect. Charge can build up on personnel and in a plant, and it can be fixed or mobile, conducting, or insulating. Charge can also build up on process materials such as insulating powders. In fact, charge can accumulate pretty much wherever there’s no direct or conducting path to ground along which the charge may flow. Accumulation needs a high (electrical) resistance to ground.
Early warning signs. Continuous charge buildup creates an unstable situation that often leads to a spark or brush discharge that we can see or hear.
Sparks: Static builds up on conductors that aren’t properly grounded. If there’s enough charge, voltage rises and can flash to another nearby conductor in the form of a spark. Sparks will typically jump a few millimeters. They can be seen from metal drums during filling or emptying and from isolated metal plant components such as pipes or conveyors, and they’re normally discrete events, meaning they occur one at a time. Sparks can also be experienced by equipment operators that have become charged. In all these cases, the sparks are energetic enough to ignite dust clouds, gases, or vapors and, if detected, must be investigated immediately.
Combining insulating materials with conductors in a plant can inadvertently lead to the conductors becoming electrically isolated from ground. A reinforcing wire in a flexible rubber or plastic hose or pipe that’s used for dust collection or powder transfer is an example of this. As powder passes through the pipe at speed, sparking can occur from the isolated reinforcing wire. The charge-generating mechanism that sparks in this way can sometimes become continuous. All metal (conductive) items in the plant must be grounded to prevent static charge buildup and sparking.
Propagating brush discharge. A propagating brush discharge occurs when there’s a charged insulator in close proximity to a conductor. This discharge can also occur when powder flows through a plastic hose or pipe or when powder collects in a plastic-lined container, hopper, or bin. The energy produced can be enough to ignite any flammable atmosphere and cause physical damage to a plant or injure plant workers. Physical signs of brush discharges include bright light flashes, noise (crack), and in the aftermath, large snowflake-like patterns on the affected insulating surface.
Brush discharges occur from insulators that have become charged, such as a charged plastic drum or powder surface. A brush discharge is limited in energy but can readily ignite common solvent vapors. Brush discharges can be difficult to detect, and safety measures must be taken to control the use of insulators in a manufacturing environment, where flammable gas, vapor, and perhaps hybrid atmospheres may be present.
A special case of brush discharge can occur across a powder’s surface as it collects in bulk in a hopper, silo, or large container. The discharge created tends to be radial in direction and multiple in frequency. This discharge type is known as a bulking brush discharge or cone discharge and can have sufficient energy to ignite more sensitive dust clouds and flammable vapors. These discharges are unlikely to be readily observed by sight or sound, although a powder mysteriously sticking to a surface indicates high charging levels and the potential for a cone discharge.
Powder handling problems: Poor flow, sieving problems, or clumping of powders can cause manufacturing difficulties, but such effects are clear indicators of static electricity generation and accumulation. You may also see powder flying away from a bulking powder during drum filling or see the powder sticking as a fine layer to container walls. If the powder is charged, so is your plant — somewhere. This is a sign to look further and assess the hazards.
Static electricity continues to cause fires and explosions in the industry. Electrostatic hazard assessments can identify static hazards and control methods. Having a specialist conduct a hazard assessment is the best way to prevent fires and explosions caused by static electricity in your facility.
Don’t miss Vahid Ebadat’s June 15 webinar “Electrostatic spark control in powder facilities: Is bonding and grounding enough?” To learn more and register, click here.
Vahid Ebadat is principal consultant and CEO at Stonehouse Process Safety.